1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly to a recording medium comprising a support having on the obverse thereof a magnetic layer and on the reverse thereof a backcoat layer, the backcoat layer being improved on the physical characteristics thereof for the purpose of improving the running stability and the S/N ratio of the recording medium.
2. Description of the Prior Art
Magnetic recording media such as magnetic tape, magnetic sheets and magnetic disks are widely used in the audio, video and computer fields. Of these fields, for example, speaking of the magnetic tape used in the video field, when performing the recording of a picture or the playback of the recorded picture, the magnetic tape contained in a cassette, after the cassette is loaded in a video deck, is run, being guided through the guide pole or guide roller, to be rubbedly scanned by a magnetic head. In this instance, for the recording of a picture or for the playback of the recorded picture, the tape needs to be run constantly and uniformly wound so as to be always ready for its repetitive use, and, since the tape is run rapidly and usually made of a high-electric-resistance material, the tape's obverse to be brought into contact with the guide pole, etc., is required to have an antistatic effect as well as to be suitable for the tape's run. Further, for the improvement of the sensitivity of the tape, particularly for the improvement of the output from the tape in high frequency regions, the surface of the magnetic layer of the tape is finished to be smooth so that the rubbing condition of the magnetic tape with the magnetic head is not varied, and is improved on the running stability and durability of the magnetic layer against the magnetic head, guide roller, and the like.
However, when the magnetic tape is run through the video deck, since not only the obverse but the reverse of the magnetic tape is also rubbed with the above-mentioned guide pole or guide roller, even if the obverse magnetic layer alone has good running stability and durability against the guide pole or guide roller, if the running stability and durability of the reverse of the magnetic tape are not satisfactory, the running magnetic tape undergoes an excessive tension to cause the magnetic layer to be rubbed to excess with the magnetic head, bringing about not only damage of the magnetic layer and exfoliation of the magnetic powder from the magnetic layer but also not-uniform tape-winding tension to vary the wound pressure to put the wound form into disorder, whereby the face formed of the wound-up edge of the tape becomes awkwardly uneven, leading to lack of the tape running speed uniformity when the tape is reused. If these phenomena occur, a skew or jitter trouble appears or the video or electromagnetic characteristics such as S/N ratios become deteriorated. In order to improve these problems, providing a backcoat layer on the reverse side of the magnetic tape is proposed.
Particularly, in recent years the magnetic recording medium such as the video tape has been improved to have a high-recording density. For performing the playback of a video image recorded on the high-density magnetic recording medium, the question of how to improve the running stability and various N/S ratios of the magnetic recording medium is a vital of importance. Upon this, some attempts have been made for the improvement.
The attempts include those relating to roughening the reverse surface of the tape and to improvement of the antistatic effect of the same. For example, there is an attempt to incorporate an inorganic powdery material into the resin backcoat layer of the magnetic tape. This is intended to roughen the surface of the backcoat layer to reduce the contact area thereof with the guide pole, etc., to lessen the coefficient of friction. For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent 0.P.I. Publication) Nos. 130234/1982, 161135/1983, 53825/1982 and 2415/1983 all describe examples of the use of inorganic powdery materials. And many of these examples define the usable particle size range of such powdery materials. However, even the backcoat layer using such the inorganic powdery material, since not only is the powdery material unable to provide a sufficient slidability but its hardness is large, when brought into contact with the guide pin, etc., scratches or scrapes off the surface of the pin, so that there are cases where the backcoat layer impairs the function of means such as the pin intended to smoothly guide and run the tape, and, besides, when the backcoat layer containing the powdery material is rubbed by such means, the combining power of its particles with the binder is not enough against the rubbing, thus tending to cause exfoliation of the particles. And when particularly large-size particles are mingled among the powdery particles, the exfoliation tends to be accelerated. Further, when the backcoat layer and the magnetic layer of the tape are brought into contact with each other when the tape is wound up, the protrusions of the backcoat layer scratch the surface of the magnetic layer or the ruggedness pattern markings of the backcoat layer are left on the surface of the magnetic layer, and such the defects, when the tape is scanned to play back a recorded video image, sometimes adversely affect the electromagnetic characteristics such as the chroma S/N ratio showing the degree of the reproducibility of the colors of the video image.
Such inorganic powdery materials as described in the above-mentioned patent publications, when used in various atmospheric conditions, tend to absorb moisture contained in air, particularly during its storage or during its use in the video deck or the like. When the magnetic recording medium thus absorbs moisute, the surface thereof becomes prone to adhere to the metallic guide pole, and the adherence may sometimes cause temporary stops of the constantly running magnetic recording medium. When the strain caused by such temporary stops becomes accumulated to excess, the recording tape is suddenly pulled this time to be fed at a great stride, causing the so-called "stick strip" phenomenon, thus resulting in the unstable running of the tape.
Instead of the above-described inorganic powery material the use of carbon black which is not as hard as the powdery material is also proposed. This is described in, e.g., Japanese Patent Examined Publication No. 17401/1977. This is intended to provide an antistatic function based on the conductivity of carbon black and the surface roughening effect by the particles thereof. However, since the mean particle size range of the carbon black used is from 0.01 to 0.02 .mu.m, the dispersibility thereof in a coating liquid thereof is very poor, and the resulting backcoat layer formed from the dispersed liquid has no uniform surface because of the aggregated particle lumps of the carbon black, which form an irregular roughness of the surface. In addition, because the combining power of the aggregated particles with the binder is not so large, they tend to be exfoliated and further larger ones of the exfoliated particle lumps leave their pattern markings on the surface of the magnetic layer to roughen the same when the tape is rewound to bring the backcoat and magnetic layers into contact with each other. After all, even when the carbon black having such a small particle size is used, it is difficult to obtain a mean surface roughness of, e.g., not more than 0.035 .mu.m, which enables to avoid the formation of the above-mentioned roughness on the surface of the magnetic layer.
As described in Japanese Patent Examined Publication No. 37003/1978, a different recording medium is also known which is such that a subbing layer is provided underneath the backcoat layer, into the subbing layer are incorporated carbon black, graphite and metallic powder, and on the subbing layer is provided another layer containing a pigment, thereby allotting the antistatic effect and surface-roughening effect to both the layers, respectively. This instance, however, has the disadvantage that, in addition to the trouble to provide double layers, the influence by the roughness according to the particles contained in the lower layer appears on the upper layer, and thus the surface is excessively roughened just as the above carbon black-applied instance is.
If the hardness of the inorganic powdery material used in the backcoat layer is thus large or the mean surface roughness of the layer is unable to satisfy the above intended value, neither the running stability nor any sufficient wear resistance and durability of the magnetic recording medium can be obtained; for example, it deteriorates the played-back video image quality, causes the magnetic layer's exfoliation trouble, makes the output fluctuation uncontrollable, or adversely affects the electromagnetic conversion characteristics such as the above chroma S/N ratio.
Particularly, the latest video recorders including VHS-type video movie, .beta.-movie and 8 mm video recorders are miniaturized to be portable with high-density recordability, so that the video recording can be made in various ways, ranging from the conventional ways of video shooting in a fixed position to the mobile outdoor shooting on the spot in various situations. Accordingly, the magnetic tape also has been desired to have characteristics durable against any atmospheric conditions. The miniaturization with high-density recordability complicates the magnetic tape run system with increasing the number of chances of the tape to touch the guide poles and guide rollers, whereby the tape is rubbed with the guide means very frequently, and therefore the improvement of the tape with respect to the wear resistance and durability to further increase the running stability and to prevent the exfoliation trouble has been demanded. In order to meet the demand, further improvement of the surface characteristics of the backcoat layer is essential.